Ultra high frequency power detecting unit



Sept. 19, 1950 o. LuNbsTRoM 2,522,525

ULTRA HIGH FREQUENCY POWER DETECTING UNIT originan FilecxmJan. 24, 1947 2 sheetssheet 1 'mf/5511er??? Sept 19, 1950 o. LuNDsTRoM I 2,522,525

ULTRA HIGH FREQUENCY `POWER DETECTING UNIT Original Filed Jan. 24, 1947 2 Sheets-Sheet 2 INVENToR 0504/? y/v0.9 mo/w Patented Sept. 19, 1950 ULTRA HIGH FREQUENCY POWER DETECTING UNIT Oscar Lundstrom, Hempstead, N. Y., assgnor to The Sperry Corporation, a corporation of Dela- Ware Original application January 24, 1947, Serial No.` 723,945. Divided and this application September 22, 1949, Serial No. 117,088

(Cl. 20L-63) 12 Claims. l

This application is a division of copending application Serial No. 723,945.

This invention relates to power measuring devices'operating at ultra-high or super-high frequencies and concerns, particularly, apparatus which may be operated successfully over a wide range of frequencies.

At ordinary radiofrequencies, the power level in a circuit may be found by measuring the voltage across or the current through an impedance which is known or measurable, but at higher frequencies, approaching the microwave region above 300 Inc/sec., accurate voltage and current meters become increasingly difficult to construct. Furthermore, at such higher frequencies most circuit elements are composed of or connected by transmission lines of appreciable electrical length, which are subject to standing waves. The voltage and current measurements must then be made at precisely the location of the known impedance or la known distance away, or values must be measured at a multiplicity of points lalong the input ltransmission line. It is, therefore, necessary at some point to turn to different methods for the measurement of power.

One such technique employed in the superhigh frequency or microwave region is the bolometric or hot-wire type of wattmeter. The heart of such a bolometric wattmeter is a sensitive resistive element whose resistance changes with s temperature and whose temperature depends upon the electrical power being dissipated in the resistance, yThe resistive element'in these measurements may take the form of a short length of fine wire. This element is placed in one arm of a direct current bridge and is heated by direct current until its resistance reaches a value that brings the bridge to balance. It is further heated bythe microwave power' which is desiredto measure, and the addition of this power causes a change in resistance which will unbalance the bridge. The unbalrance indication in the gialvanometer of the bridge may then be used to measure the amount of microwave power.

Such bolometers are usually connected into the transmission line carrying the power to be measured, by the kuse of a special holder adapted for this purpose. If the hot-wire clement is to absorb substantially all the power in the incident wave it is necessary that the bolometer and its holder be well matched to the input transmission line. This matched condition must exist in the entire frequency range over which the wattmeter is to be operated.

It isr also advantageous to have a bolometer holders, as this simplifies the replacement problem. A further advantage is realized if the holder is of the non-tunable broad-band type. This permits replacement of the bolometer without requiring retuning of the holder to maintain the 4desired high-frequency impedance properties,

Simplicity and flexibility of operation also de-v mand that the desired impedance properties be maintained over the widest possible range of frequencies. v

It is, therefore, yan object of this invention to provide an improved hot-wire bolometer element and an improved holder for such hot-Wire element having some or all of the above-discussed desirable qualities.

Another object of the invention is to provide a concentric line hot-wire holder which may readily be opened for replacement of the hot-wire element.

Another object is to provide a hot-wire holder which will match the hot-wire element to the concentric transmission line over a Wide range of the stated objects, or in the stated fields or combinations.

A still further object of the invention is to prol vide a broad-band wattmeter Awhich eliminates the necessity for tuning the holder while permitting operation over la wide range of frequencies.` Briefly, a Wattmeter constructed in accordance with this invention has the hot-wire bolometer element inserted at the end of a section of coaxial line, the element itself being essentially an eXtension of the inner conductor with the other end of the wire terminating on a coaxial line shorting member serving to provide la short-circuiting termination for said line. Mounting the element adjacent a coaxial line short guarantees its being positioned at a current loop, thereby providing more accurate power measurements. If the hot-wire is operated at a resistance near the characteristic impedance of the input line, the bolometer element in la fixed tuned holder will present a suitable matched load to the input line from a very low frequency up to a frequency at which the reactance of the wire reaches an ap-V preciable magnitude. The high frequency limit may be further extended according to the invention by Iusing the discontinuity capacity effect at the point 'where the wire is attached to the center conductor to resonate the wire inductance,

and by using a hot-wire resistance denitely diiferent from the input line characteristic impedance.

For a better understanding of the present invention, together with other and further objects thereof, reference is added to the following description taken in connection with the accoinpanying drawings, where Fig. l is a diagrammatic sketch of a hot-wire type bolometer element connected to an ultrahigh-frequency generator and input transmission line and to the power-measuring bridge circuit showing the system in which the present invention is used;

Fig. 2 is a longitudinal cross-sectional prospective view of the bolometer element constructed in accordance with this invention;

Fig. 3 is a longitudinal cross-sectional view of the end of an input coaxial line with the bolometer element of Fig. 2 inserted, showing the bolometer element holder of the present invention;

Fig. 4 is a schematic longitudinal cross-section of a coaxial line terminated in a hot-wire element, useful in explaining the theory behind the present invention;

Fig. 4A is a circle diagram showing the impedance relations which exist in a conventional device;

Fig. 4B is a plot of the variation of voltage standing wave ratio with frequency;

Fig. 5A is a circle diagram showing the impedance relations which exist in the device shown in Fig. 4;

Fig. 5B is a similar admittance circle diagram, and

Fig. 5C is a plot o the variation of voltage standing wave ratio with frequency.

Referring now more particularly to Fig. l there is shown a diagrammatic sketch of a hotnriretype bolometer element, as `in the present invention, connected to a power measuring bridge circuit. A generator I0 which supplies ulta highor super-high-frequency energy, whose power level is to be measured, is connected to a transmission line II. Hot-wire element I2 is connected in series with one of the legs of transmission line I I, and a by-pass condenser I3 serves to complete the alternating current circuit. Bridge circuit |4- is of the conventional Wheatstone type having resistors I8, I9 and 2l! as three arms thereof, hot-wire I2 forming the fourth arm. Battery1 2| is connected across one diagonal of the bridge circuit I4 and supplies direct current thereto through an adjustable resistor 22 which is used to control the amount of current supply. Galvanometer G is connected to the other bridge diagonal and is used to measure bridge unbalance. rlhe direct current path in the fourth arm connected between terminals 2 and 28 of bridge I4 may be completed by a conventional stub support along transmission line I I (not shown) or may be made through the generator Il) at the end of transmission line II.

Briey, in operation, bridge circuit I4 is balanced with generator I0 in a non-operative condition. Current supplied by battery 2| is used to heat element I2, changing its resistance and thereby balancing the bridge. Balance may also be adjusted by varying resistor 20. As microwave energy is transmitted along line I I by generator IG, the hot-wire element I2 is further heated by the microwave power. The heating eiect of the microwave power causes the hot-wire element I2 to change its resistance Cil which will, in turn, unbalance the bridge circuit I4. The unbalance indication in galvanometer G may then be used to measure the amount of microwave power being transmitted down line II. Calibration may be accomplished by rst balancing the bridge with direct current and then noting the unbalance caused by adding a known amount of low frequency power to the sensitive element, or else the bridge may be balanced with direct current alone, and then rebalanced after addition of the microwave power. The microwave power is then said to equal the amount of direct current power that it was necessary to subtract from the bolometer in order to regain the balanced condition.

Fig. 2 shows the bolometer element 29 constructed lin accordance with the present invention. The hot-wire I2 is suspended between and along the axes of two metallic disc members 30 and 3| which are separated by and mutually vacuously sealed to glass cylinder member 32. Disc 30 has a diameter equal to the inner conductor of the coaxial transmission line which the bolometer 28 is to terminate. A prong 33, extending along the axis of disc 30, is adapted to be inserted into the end of the coaxial transmission line so as to center the unit and to provide good electrical contact and mechanical stability thereto. Glass cap 34 sealed to the other side of disc 3| is used in sealing off the evacuated chamber surrounding element I2, holes 25 in disc 3| being provided to permit evacuation of this chamber. The diameter of disc 3| is somewhat greater than that of the disc 3U, and it is preferably so thin as to bc exible, for purposes discussed below.

Fig. 3 shows the bolometer element 28 terminating a coaxial line 4|! having inner conductor 4| and outer conductor 42. Outer conductor 42 terminates in an inner flange portion 43 having a tapered bore. Bucket-shaped member 44, having a tapered outer wall, is fitted into tapered flange member 43, dielectric tape 45 being inserted between members 43, 44 and serving to insulate them electrically from one another. A circular opening 46 `is provided in the center of dat end portion 41 of member 44 to permit the insertion of bolometer element 29. Prong 33 ilts into an axial bore 48 in the butt end of inner conductor 4I permitting disc 30 to make ilush contact with the end of the inner conductor 4|. The thickness of the end portion 41 of member 44 is made substantially the same as the length of glass cylinder member 32 of bolometer 29. This permits disc member 3| to make contact with the inner face of the end portion 4T. Cylindrical plug member 50, having a threaded outer wall, engages threads 5| formed on the inner wall of member 44 and clamps disc 3| between it and end portion 4T to provide good contact therebetween as well as between disc 3l and the butt end of inner conductor 4 The flexible character of disc 3| permits self-alignment if any slight inaccuracy of dimension exists.

'Ilhe complete bolometer holder assembly is so designed so that the point at which element I2 attaches to disc 30 is substantially in the plane of the outer face of end portion 41 of member 44 which, in turn, coincides with the inner end of ilange member 43. The result of this geometry is a sharp decrease in the inner diameter of the outer conductor 42 at the same point along the coaxial transmission line 40 as the decrease in the diameter of the inner conductor 4 I.

Electrical connection is made to outer cons. tdu'ctor 42 -bylead`wire28 and'to member by lead wire 21. These lead wires are connected to `4 terminals 21 and 28 of bridge circuit I4 .as :shown schematically -inFig 1, and power measurements are made in the manner described above.

`As pointed out above, if the hot-wire element l2 isto absorb substantially all thepower in the .incident wave, it is necessary that thebolometer 'Z9-fand its `holder' assembly be matched to the sectiono'f transmission line 40 A'which it terminates. Otherwise, a fra'ction of theincident power will be reflected and the resulting power indication will of necessity be lower than the amount of incident power transmitted along the line 40 which is tobe measured.

'The reilected energy creates standing waves `in thetransmission line. By measuring the ,ratio of the amplitudes `of the volt-age loops or tanti nodes to the amplitude of the voltage nodes, figure representing the matched condition is yobtained. Thisisreferred to in the artas the voltyage:standing wave ratio and is abbreviated as VSWR. In designing microwave power transmission equipment, it is desirable to kkeep the VSWR minimizedso that little power is lost by reflection.

Theamountof mismatch (the magnitude of the VSWR) that is permissible depends, of course, upon the use to which the wattmeter is to be put. However, for great precision in power measurements, the VSWR must be kept .to a small value. `It is not necessary that it be the same at all frequencies, but it is ynecessary that it shall not exceed the design value at any point in the range of Ifrequencies over which the .wattmeter is to measure power.

By making the wattmeter of the tunable type, itis possible to obtain a low VSWR at a given ireN quency, but if the operating frequency is changed, the device must be retuned in order to maintain the low-VSWR. Even in such a tunable device, however, there are tuning limits beyond which it is not possible to maintain the .desired VSWR.

'Just how the VSWR changes with frequency ,in a particular device may be seen rby reference toFigures 4A and 4B. Fig. 4. is alongitudinal cross-section of a section of coaxiall line 6! having an inner conductor 6l and an outer conductor 62. Resistor element l2', which may be the hot- -wire of the invention, connected to the butt end of inner conductor 6l, terminates upon shorting member 3 I which seats upon Aand closes the butt end of outer conductor 62.

Fig. 4A is a conventional impedance or circle diagram which is useful in explainingthe operation of the device shown in'lig. 4. A discussion of the use of such circle diagrams is given in chapter 8, page 55, of the book Principles of Radar written by the MIT Radar School sta-iiF and Hpublished by McGraw-Hill, 1946. 'The diagram in Fig. 4A can be used to show how the impedance of element l2 varies with operating frequency and further to show the condition of mismatch that exists between transmission line'secvtion 60 having El as its inner conductor and the transmission line section having element l 2 as its inner conductor.

In Fig. 4A, the abscissa axis represents a normalized resistance R equal to the resistance oi element I2 coupled to line 60, divided by the :characteristic impedance Ze of line Et. Similarly, the ordinate axis represents the normalized reactance X equal to the reactance element l2 divided by Ze. Point (l having coordinates (L) represents the `characteristic impedance Ze.

A series of circles C1, Cris superimposed upon the usual rectangular coordinates. These circles have their centers on the laxis of abscissae and through points on therabscissae axis having reciprocal abscissae values. As plotted, each o f these circles represent the locus of impedance Values which, when coupled at the end of line 60, will produce a constant mismatch condition and, as such, may be referred to as constant voltagel.standing-waveratio circles. For illustration, circle C1 may represent a VSWR of 1.5, and C2 a VSWR of 2.0.

If element I2' is designed to have an ohmic resistance value equal to the characteristic impedance of linef'BO, its normalized value R would be represented by point Le on the circle diagram. However, if the device is to be used vat high frequenciesthe element l2 will begin to exhibit its inductive properties with rthe result that itsimpedance ceases to be pure resistance but has an inductive reactive component which increases with'the operating frequency. The resuting impedance locus of the'element I2 withvarying frequency is then the vertical line 0W Von the graph. It will be noted that the resistive component does not change, but that the reactive component increases. This is due, of course, to the increased inductive reactance `which element l2 exhibits as the operating frequency is increased.

If the operating frequency is extremely low,.the impedance of element l2 approaches a pure resistance chosen equal to the impedance ofthe transmission line Sil itself. This, of course, represents a matched condition and allows for the maximum possible transfer of power. However, as the Vfrequency is increased and the hot-wire impedance begins to have a positive reactive component, a certain amount of mismatch occurs. At point A on Fig. 4A there is indicated by the length UA the maximum amount of inductive reactance which element l2 may exhibit and still have a VSWR less than 1.5. As the frequency is increased and the resulting inductive reactive component increases to point B, it is seen that lthe standing-wave-ratio has now reached the value of 2.0. It is therefore seen thatfor a given vpermissible standing wave-ratio there is an upper operating frequency limit at which this standing wave ratio is reached. The tangent of the angle 61 is proportional to the operating frequency at point A.

Fig. 4B is aplot or the voltage standing wave ratio against the impedance of element l2', which, in turn, is a function of the operating frequency. It will be seen that at point il the standing wave ratio is unity. This, Aof course, is the perfect match condition. As the impedance in; creases to point A, the volta e standing wave ratio has now reached a value equal to 1.5 Yand a further increase in the impedance of element l2 to point B causes the VSWR to reach a value bf2.

In order to increase the range of frequencies lover which the hot-wire element I2 is matched .within a permissible VSWR, such as 1.5, reference is made to Fig. 5A., which is another circle diagram representing the impedance charactern istics `of the device shown in Fig. 4. In Fig. 5A the resistance of element I 2' is no longer taken vas equal to the line characteristic impedance ZC,

but is reduced byfaiiactor equal to the reciprocal of ,themaximurn allowable VSWR, which is taken to ybe 1.5 'for rthe present example. Therefore,

the ohmic resistance of element 12 is represented by point A', having coordinates By so reducing the resistance it can be seen that, as the operating frequency is increased, the presence of any reactance in the element i2 immediately causes the VSWR to exceed the permissible value 1.5, since the locus of the impedance of element i2 now is line AW.

Fig. B is an admittance diagram equivalent to the impedance diagram of Fig. 5A. Each point in Fig. 5B is the inversion of the corresponding point of Fig. 5A, corresponding points being designated by the same letter. Thus, circles C1 and Cz are the same in the two diagrams. The normalized ohmic conductance of element I2' is given by point A' of Fig. 5B, having coordinates (l.5,0). The line A'W' of Fig. 5A, representing the locus of the impedance values of element l2', becomes semi-circle A'W of Fig. 5B. It is immediately seen that all points on the admittance curve A'W except A' fall outside of the VSWR:1.5 circle.

By adding a positive susceptance of sufficient value to the admittance of element l2', a large portion of the resultant admittance locus curve will then fall inside the 1.5-VSWR circle. The addition of such a susceptance (by coupling a capacitive element in parallel with element I2) permits the input admittance to line Bil to become real at one frequency in addition. to "zero frequency (point A') According to an important feature of the present invention, the proper susceptance is added to produce as wide a frequency range as possible, without exceeding the permissible VSWR value.

Referring to Fig. 5B; point A" represents the inverse of point A', still on the circle C1 of permissible VSWR. Now, if susceptance A"D is added to the circuit, point D of the semi-circle A'W will be moved to point A", and the input admittance at the frequency corresponding to point D will be a pure conductance. As at point A', the standing wave ratio will be 1.5. The addition of the susceptance A"D will bring all points on the admittance curve between A and D inside the VSWR=1-5 circle. Reference to Fig. 5C shows a plot of the VSVVR against frequency. It is seen that the value of the VSWR is 1.5 at points A and D. Between these points it is always less than the allowed Value 1.5. The increased frequency range brought about by the addition of the susceptance can be seen by comparing the magnitudes of 01 and 62, the tangents of their angles being proportional to the upper operating frequency limits.

The susceptance which must be added to the admittance of element l2 should have a positive value, that is, it should be a capacitive susceptance. According to the present invention, the capacity which is added in parallel with the line is in the form of a so-called discontinuity capacity. A complete treatment of discontinuity capacity is presented in an article entitled "Coaxial-Line Discontinuities by Whinnery, Jamieson and Robbins, appearing in the November 1944 issue of the Proceedings of the IRE. In this paper, it is shown that the effect of certain steptype discontinuities in coaxial transmission lines is the same as the placing of an admittance between the inner and outer conductors of the transmission line at the point of the discon- 8 tinuity. It is further pointed out that if the transverse dimensions of the discontinuity are a small fraction of a wavelength at the operating frequency, the discontinuity admittance is purely capacitive.

The required discontinuity capacity is provided by element 13, in Fig. 4, which causes a discontinuity in the outer conductor to occur at the same point along the transmission line as the discontinuity introduced by the change in the inner conductor where element I2' joins inner conductor G l, and determines the magnitude of the total capacitive susceptance introduced in the line, since the discontinuity introduced at the inner conductor is fixed by the necessary size of the hot-wire I2 required for bridge I4. Formulas are given in the paper referred to for calculating the required physical discontinuity necessary to add a desired amount of susceptance. VVThe magnitude of the susceptance is, of course, obtained from a circle diagram such as Fig. 5B.

In Fig. 3, nat end portion 47 serves the same function as cylinder '13 in Fig. 4, namely, to supply the outer conductor discontinuity and thereby introduce a given amount of capacitive susceptance across the line at the point where hotwire element l2 joins inner conductor 4|.

From the above discussion it is thus possible to design a bolometer element and holder which will give the maximum operating range of frequencies without exceeding a permissible voltage standing wave ratio. Since the resistance of the element alone determines the impedance mismatch with the transmission line to which it is connected, its value is selected so that the VSV/'R at zero frequency equals the permissible value. Thus the resistance of the element is set at where a is the permissible VSWR and ZC is the characteristic impedance of the transmission line. The material and dimensions of the element are selected to give the necessary resistance value, consideration being given to the amount of power which the element must dissipate.

The resulting element is in the shape of a rodlike filament and as such will exhibit inductive properties. This causes the impedance properties of the element to have a reactive component whose magnitude increases with increased operating frequencies. Since the VSWR presented by the mismatch between the transmission line and the hot-wire element equals the maximum permissible value at zero frequency, as the operating frequency is increased, the VSWR will be increased due to the addition of inductive reactance to the resistance of the element.

By the addition of capacitive susceptance in parallel with the bolometer element it is possible to electrically cancel the inductive reactance at some high frequency so that the element will present an impedance which is a pure resistance as at zero frequency. The VSWR at this high frequency will again be the maximum permissible value, and will be less than the maximum permissible value at all operating frequencies between zero at the upper limiting operating frequency.

Knowing the inductive properties of the element, which can be determined from its physical dimensions, it is possible by the use of circle diagrams to determine both the magnitude of the capacitive susceptance which must be added and the upper operating frequency. The desired amount of capacitive susceptance may then be built into the holder by applying the formulas as given by Whinnery.

This procedure gives the steps in the design of a bolometer element and holder which has the broadest range of operating frequencies without exceeding a permissible VSWR and gives wide latitude in the selection of the element material in view of the power dissipation requirements.

The present invention, therefore, provides a bolometer or hot-Wire element in a coaxial line section adjacent a line short. It further provides a bolometer holder which permits rapid and easy interchange of bolometer elements. Further, by designing the bolometer holder so that it introduces a predesigned amount of discontinuity capacity, a bolometer element is provided which opcrates over a wide range of frequencies without exceeding a fixed VSWR. The operating band of frequencies has no lower limit, inasmuch as the permissible VSWR is not exceeded at any operating frequency between the upper operating limit down to direct current.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made Without departing from the 'scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A bolometer adapted to be coupled to a coaxial transmission line comprising a glass cylindrical member, a iirst disc-shaped conductive member sealed to and in alignment with said glass cylindrical member and forming a flushedged end wall thereof, a second disc-'shaped flexible conductive member sealed to and in alignment with said glas-s cylindrical member and forming the opposite end wall thereof, said second member having a diameter greater than said glass member thereby forming a exible metal flange extending beyond the external wall of said glass member and adapted to be coupled across the end of the outer conductor of said line, a filament member supported along the axis of said glass member and having its ends conductively connected to said rst and said second conductive member respectively, a cylindrical prong member rigidly fastened perpendicularly to the external face of said first disc member and in alignment therewith and adapted to be received within the inner conductor of Said line to cause said iirst member to form a substantially smooth continuation of said inner conductor, and an evacuating glass cap member sealed to the external face of said second disc member.

2. A bolometer comprising a glass cylindrical member, a iirst metallic disc member sealed to one end of said glass member and in alignment therewith, said rst disc member having a diameter substantially equal to said glass cylindrical member, a second metallic disc member sealed to the other end of said glass cylindrical member and in alignment therewith, said second disc member having a diameter greater than that of said cylindrical member, and a filament member supported along the axis of said glass cylindrical member by said first and said second disc members.

3. A bolometer comprising an evacuated charnber, a filament supported inside said chamber, a pair of conductive disc-shaped end plate members electrically connected to opposite ends of said illament and forming opposite walls of said evacuated chamber, said walls being parallel and said discs being aligned.

4. A bolometer comprising an evacuated cylindrical chamber, a first metallic disc member sealed to one end of said chamber and forming and end wall thereof, said lirst disc member having a diameter substantially equal to that of said cylindrical chamber, a second metallic disc member insulated from said rst member and sealed to the other end of said chamber in alignment therewith and forming an opposite end wall of said chamber, said second disc member having a diameter greater than that of said irst disc member, and a lament member within said chamber and extending between said iirst and second disc members.

5. A bolometer comprising a dielectric cylindrical member, a conductive member sealed to one end of said dielectric member, a metallic disc member sealed to the other end of said dielectric member, Said disc member having a diameter greater than that of said cylindrical member, and a filament member supported along the axis of said cylindrical member by said conductive member and said disc member.

6. A bolometer as in claim 5 wherein said disc member is in alignment with said dielectric member.

7. A bolometer as in claim 5 wherein a prong member is supported by said conducting member.

8. A bolometer as in claim 7 wherein said prong member is oriented coaxial with said dielectric cylindrical member.

9. A bolometer comprising an evacuated chamber, a filament supported inside said chamber, a conductive end member electrically connected to one end of said filament, a conductive disc-shaped end member electrically connected to the other end of said lament, the diameter of said discshaped end member exceeding the maximum dimension of said evacuated chamber.

10. A bolometer comprising an evacuated chamber, a filament supported inside said chamber, a pair of conductive disc-shaped end plate members electrically connected to Opposite ends of said filament and forming opposite walls of said evacuated chamber, one of said disc-shaped members having a diameter greater than the maximum dimension of said evacuated chamber.

l1. A bolometer as in claim 10 wherein said disc-shaped members are aligned and parallel.

12. A bolometer comprising a dielectric envelope having a cylindrical shape, a rst conductive member sealed to one end of said dielectric en velope, a second conductive member sealed to the opposite end of said dielectric envelope, a flange member extending radially from said dielectric envelope, said ange member being conductively connected to said second conductive member, and a filament member supported along the axis of said dielectric envelope, having its ends conductively connected to said first and said second conductive members.

OSCAR LUNDSTROM.

REFERENCES CITED UNITED STATES PATENTS Name Date Webber July 29, 1947 Number 

